In compounds

As a consequence, this process took place also in Old Sindarin compounds, when the initial letter of a word was no longer initial in a compound. Even in later compounds that would be pure Sindarin, the lenition was solidified as a rule, envisioning retroactively the lenition process from a theoretical Old Sindarin compound.

For example the word athrabeth "debate, converse (lit. 'across-word')" can be analyzed both as a derivative of OS word *athrapettha but also as a compound of Sindarin words athra- "across" + peth "word".

The fact that the above sounds mutated when medially was solidified as a rule